One of the many unfortunate aspects of arguments over climate change is that it's where many people come across the idea of a scientific consensus. Just as unfortunately, their first exposure tends to be in the form of shouted sound bites: "But there's a consensus!" "Consensus has no place in science!"

Lost in the shouting is the fact that consensus plays several key roles in the process of science. In light of all the consensus choruses, it's probably time to step back and examine its importance and why it's a central part of the scientific process. And only after that is it possible to take a look at consensus and climate change.

Standards of evidence

Fiction author Michael Crichton probably started the backlash against the idea of consensus in science. Crichton was rather notable for doubting the conclusions of climate scientists—he wrote an entire book in which they were the villains—so it's fair to say he wasn't thrilled when the field reached a consensus. Still, it's worth looking at what he said, if only because it's so painfully misguided:

Let's be clear: the work of science has nothing whatever to do with consensus. Consensus is the business of politics. Science, on the contrary, requires only one investigator who happens to be right, which means that he or she has results that are verifiable by reference to the real world. In science consensus is irrelevant. What is relevant is reproducible results.

Reproducible results are absolutely relevant. What Crichton is missing is how we decide that those results are significant and how one investigator goes about convincing everyone that he or she happens to be right. This comes down to what the scientific community as a whole accepts as evidence.

In an earlier discussion of science's standards for statistical significance, we wrote, "Nobody's ever found a stone tablet etched with a value for scientific certainty." Different fields use different values of what they think constitutes significance. In biology, where "facts" are usually built from a large collection of consistent findings, scientists are willing to accept findings that are only two standard deviations away from random noise as evidence. In physics, where particles either exist or don't, five standard deviations are required.

While that makes the standards of evidence sound completely rational, they're also deeply empirical. Physicists found that signals that were three standard deviations from the expected value came and went all the time, which is why they increased their standard. Biologists haven't had such problems, but other problems have popped up as new technology enabled them to do tests that covered tens of thousands of genes instead of only a handful. Suddenly, spurious results were cropping up at a staggering pace. For these experiments, biologists agreed to a different standard of evidence.

It's not like they got together and had a formal vote on it. Instead, there were a few editorials that highlighted the problem, and those pieces started to sway the opinions of not only scientists but journal editors and the people who fund grants. In other words, the field reached a consensus.

That sort of thing is easiest to see in terms of statistical significance, but it pervades the process of science. If two closely related species share a feature, then we conclude it was present in their common ancestor. The scientific community decided to establish 15 percent ice coverage as the standard for when a region of the ocean contains ice. It required that every potential planet imaged by the Kepler probe must have its presence confirmed by an independent method before being called a planet. There's no objective standard that defines any one of these test as the truth; it's just that the people in the field have reached a consensus about what constitutes evidence.

Consensus is not just for standards

Just as fields reach a consensus about what constitutes evidence, they reach a consensus about what that evidence has demonstrated. Confusion about the potential causes of AIDS dominated the early years of the epidemic, but it took researchers only two years after the formal description of the disorder to identify a virus that infected the right cells. In less than a decade, enough evidence piled up to allow the biomedical research community to form a consensus: HIV was the causal agent of AIDS.

That doesn't mean that every single person in the field had been convinced; there are holdouts, including a Nobel Prize winner, who continue to argue that the evidence is insufficient. Those in the field–and humanity in general—simply don't find their arguments persuasive. We've since oriented public policy around what the vast majority of experts consider a fact.

In most fields, however, the stakes aren't quite so high. You get informal consensuses forming around things that the public isn't ever aware of: the existence of morphogens in patterning embryonic tissues, the source of the radiation in the jets of quasars, and so on. If you asked a large group of scientists, their consensus would be that consensus is a normal part of the scientific process. Contrary to Crichton's writings, the consensus forms precisely because reproducible evidence is generated.

Consensus matters

On its own, the existence of a consensus seems trivial; researchers conclude some things based on the state of the evidence without that evidence ever rising to the level of formal proof. But consensus plays a critical role in the day-to-day functioning of science as well.

In The Structure of Scientific Revolutions, Thomas Kuhn discussed the idea of paradigms: big intellectual frameworks that organize a field's research. Paradigms help identify problems that need solving, areas that still have anomalous results—all while giving researchers ways of interpreting any results they get. Generally, they tell scientists what to do and how to think of their results. Although not as important or over-arching as a paradigm, a consensus functions the same way, just at a smaller scale.

For example, researchers will necessarily interpret their results based on what the consensus in their field is. So odd cosmic observations will be considered in terms of the existence of dark matter particles, given that there's a consensus that the particles exist. It doesn't matter whether the researchers—or their results—agree with the consensus. The existence of a consensus simply shapes the discussion. In the same way, research goals and grants are set based on areas where the consensus opinion seems a bit weak or has unanswered questions.

At first glance, this may seem like it can stifle the appearance of ideas that run counter to the consensus. But any idea in science carries the seeds of its own destruction. By directing research to the areas where there are outstanding questions, a consensus makes it more likely that we'll generate data that directly contradicts it. It may take a little while to get recognized for what it is, but eventually the data will win out.